[ADI][unit test] Remove unused constant and add null checks in DeveloperVerifierControllerTest.

The `TEST_VERIFIER_COMPONENT_NAME` constant is removed as it is no longer referenced. Assertions are added in `setUp` to ensure the verifier package name is not null after initialization.

BUG: 399436145
Test: abtd
FLAG: android.content.pm.verification_service
Change-Id: I642ae3532c803fbab76f7a3c527dbf3a55ccf776